The invention relates to a changeable sun and planet gear mechanism. More particularly, a sun and planet gear transmission mechanism is used for power wrenches and comprises a power input shaft and at least two successive gear steps of a sun gear and at least one planet gear.
The use of several successively arranged gear steps and a sun and planet gear arrangement is well known for obtaining large transmission ratios. In such arrangements, the driven power input shaft has a sun gear disposed at the end thereof in the first gear step. The planet gears are supported by a planet gear carrier and are rotatively mounted in the gear casing. The gear casing has an internal toothing disposed along its entire periphery on the inside surface thereof. The input torque is transmitted from the power driven input shaft to the planet gears and the planet gear carrier via the sun gear located in the first gear step. Upon transmission of the input torque, the planet gears rotate in the gear casing and effect a roll motion at the internal toothing of the gear casing.
The planet gear carrier of the first gear step is connected anti-rotationally with the sun gear of the second gear step or it is formed in accordance therewith. The planet gears of the second gear step rotate around the sun gear of the second gear step. Depending upon the desired transmission ratio, any number of further gear steps may be located within the sun and planet gear mechanism. It is known to dimension the individual gear steps in such a way that a through internal toothing can be provided in longitudinal direction at the gear casing. The planet gears of all the gear steps engage this internal toothing.
It is also known to use multi-stage sun and planet gears for power wrenches in which the torque to applied manually and transmitted with amplification to a screw head coupled with the power output shaft. To accomplish this, it is necessary to deviate the reaction force of the gear case to prevent the gear case from rotating with the shaft. Therefore, it is known to fit a bow at the gearbox finding its support near the screw at a fixed point.
The transmission ratio may be changed for prior art gear wrenches. Claws are provided at the coupling portion that are connected with the power input shaft. These claws engage counter-claw of the gear case if the power input shaft is inserted in longitudinal direction into the gear case. When the arrest is effected this way, the gear is blocked and the screw can be turned only if the total case co-rotates without the mutual relative movements of its members. Therefore, it is possible to change between the normal transmission ratio and a transmission of 1:1.